I am not adding anything positive, I just saw 300 posts and thought I was missing the biggest fight in Pinside history.

Carry on

Replace those caps....the good ones read like mine

Or swap a good and bad and see if problem moves

Good idea on replacing them as a set....When you do that and plug everything back in, I would do it with the three grounds removed from those coils. Then plug J9 in with the grounds off the coils and make sure the FETs aren't turned on. Then attach one ground at a time and test as you go. That way you'll know if there's something on the coil side taking out the bank of components.

You are correct on the caps.

Anything's possible at this point. Doesn't hurt to try. If you had a molex pin remover, I'd say pull every pin out off of J9 and start inserting them one at a time into the connector and plugging it back in to see when/if the FETs turn on.

robert, I believe that J9 is an IDC connector. No pins or terminals to remove.

Ah, you're right....my mind's going considering I just unplugged my J9 today to make comparable readings

Robert I was thinking about that a few days ago. Since J9 is an IDC then Apollo could desolder the coil wire that is going to J9 from the 3 coils. Apollo, do you have any alligator clips or test leads?
You could tack solder the alligator clips to the 3 wires or you could use test leads to connect each coil one at a time back to the J9 return wire for each coil. We can see if one, two or all 3 lines cause the mosfets to have positive voltages on the gates. Basically what robert was stating.

Now this brings me to the question of if we tested all the J9 coils for ohms. Not sure if we did this or not. Using your DMM set to the lowest Ohm scale, connect one lead to the 50 VDC line and then one at a time measure each pin on J9. You should see no shorts but you should read a low resistance. Have we done this or can we do this?

Apollo are you also saying that you think Q12, 13 and 14 are blown again?

That's true....Just please put a wire nut on them with tape, so none of them touch voltage and blow up something more

He's had the three coil grounds off and J9 plugged in, but I can't remember if he measured the FETs during that time period. The three coils measure good because we measured drain to drain on the FETs which was measuring two coils at a time in parallel. I'm wondering if one of the 'good' coils is the culprit. I like your idea of measuring ALL the coil resistances and I would go the extra step of removing all the coil grounds at the coil and plugging J9 in. With no grounds connected at the coils, we're first seeing if some stray short is on one of the wires. See if there is anything on the gates then. If no, then start reconnecting one at a time, starting with the good ones. After each reconnect, check the FET voltage again. Tedious, but it may enlighten us.

Yeah, start there....with the three off, see if you have voltage on those 'bad' fets.

Apollo are these the J9 return wires from the 2 slings and the VUK coils?
If these are disconnected, what is the Gate to Gnd DC voltage when J9 is connected?
The gate voltages should be ~zero.

If they are then reconnect the coil return wires one at a time.
Let's say we start with the left sling return going to J9-5 first. Measure the 3 gates and see what we have with that coils return wire connected. If we have zero then reconnect the next coil, right sling, J9-6.
If we have positive voltage on the gates when J9-5 is connected, disconnect that line and then connect J9-6 and then measure the gates.

What we want to do here is isolate each return and see what happens as each is reconnected.

Check with just the three off first as pdman suggests...no need to remove them all yet. If you get to the point of removing them all, then:

While you're lifting each ground, once the ground is off, check the resistance of the coil like pdman was asking.

So, order of operation:

Lift ground, tape it good
Check Ohms of coil
Repeat for all of them

With J9 plugged in, turn on machine, check for VDC on the Fets

If none, machine off, reattach one ground, starting with working coils

Repeat previous step

Definitely this is what we need to do. It will show us exactly when Q12, 13 and 14 get the positive voltages on their Gates!!!

OK, remove the P6 wire and do P5 and P7.

When you say 0 vdc are you checking all the FETs? The way you have it listed sounds like you're only checking the vdc on the FET you're reattaching each time. The goal was to check the three bad FETs every time you reattached a new ground to see if they ever came on. We already know all the other FETs stay at 0.

I can't follow what you just wrote...

Ok, I think I see....You left out connected '5' and connected '7'

Okay....so no one coil is causing another. They're limited to each other.

Okay, do this....remove those three bad ones

Remove one good one.

Get a length of wire to bridge from the good coil to the bad coil

Connect the good coil ground to a bad coil wire/pin using the jumper. That way we isolate coil or board again. If the good coil on the bad pin still causes that pin to lock on, then it is still board. If not, then it's the coil.

You can then use that same jumper in the opposite direction. Connect a bad coil to a good pin.

Good Call!!

I expected as such but needed to exhaust all possibilities. The only other thing to check is the wire itself as swapping them still used the bad pin wire. You can pull J9. Get a length of wire and solder one end to a coil ground. With game on, touch the wire to each J9 pin. Coil will probably come on with the three bad ones. If it doesn't then maybe you've got something shorted in the wiring bundle somewhere.

Yep....Probably those caps.

don't see how the caps would turn on the mosfets.
Anyways apollo you did a lot today.
I'm gonna digest what the results were tonight and tryand figure out what can be a logical explanaation.

No worst case you send it to Clive or Rob Anthony and for $85 bucks and they repair it. You don't need a new one.

Me either, but they are the only devices that measure non-spec. With U2 removed, the resistors surely aren't going to turn them on. The Gates are tied to the caps across ground. If a cap opens up under load, then maybe something going on there. I'm 25 years removed from my TTL/transistor classes, so I don't remember alot, but if those FETs have 50V on the drain which they do through the coil resistance, and the gate is floating, and the source is grounded, there may be some reverse saturation going on....The potential between the gate since it is floating and the source which is grounded is not 0V and so the FET turns on. It only happens when there is the 50V on the drain.